Filter for filtering particulate matter from exhaust gas emitted from a compression ignition engine

ABSTRACT

A filter for filtering particulate matter (PM) from exhaust gas emitted from a compression ignition engine, which filter comprising a porous substrate having inlet surfaces and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a porous structure containing pores of a first mean pore size, wherein the porous substrate is coated with a wash coat comprising a plurality of solid particles comprising a molecular sieve promoted with at least one metal wherein the porous structure of the wash coated porous substrate contains pores of a second mean pore size, and wherein the second mean pore size is less than the first mean pore size.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional application of the U.S. applicationSer. No. 13/203,631 filed Jan. 6, 2012 now U.S. Pat. No. 8,608,820 whichclaims priority to U.S. National Phase application of PCT InternationalApplication No. PCT/GB2010/050347, filed Feb. 26, 2010, and claimspriority of British Patent Application No. 0903262.4, filed Feb. 26,2009, and British Patent Application No. 0922612.7, filed Dec. 24, 2009,the disclosures of all of which are incorporated herein by reference intheir entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to a filter for use in treatingparticulate matter (PM) and oxides of nitrogen derived from acompression ignition engine.

BACKGROUND OF THE INVENTION

Compression ignition engines cause combustion of a hydrocarbon byinjecting the hydrocarbon into compressed air and can be fuelled bydiesel fuel, biodiesel fuel, blends of diesel and biodiesel fuels andcompressed natural gas. The purpose of the present invention isdifferent from the invention claimed in UK patent application no.1003244.9 filed on 26 Feb. 2010 entitled “Filter”. The purpose of theinvention in that patent application is a filter for particulate matterin exhaust gas of a positive ignition engine.

Ambient PM is divided by most authors into the following categoriesbased on their aerodynamic diameter (the aerodynamic diameter is definedas the diameter of a 1 g/cm³ density sphere of the same settlingvelocity in air as the measured particle):

(i) PM-10—particles of an aerodynamic diameter of less than 10 μm;

(ii) Fine particles of diameters below 2.5 μm (PM-2.5);

(iii) Ultrafine particles of diameters below 0.1 μm (or 100 nm); and

(iv) Nanoparticles, characterised by diameters of less than 50 nm.

Since the mid-1990's, particle size distributions of particulatesexhausted from internal combustion engines have received increasingattention due to possible adverse health effects of fine and ultrafineparticles. Concentrations of PM-10 particulates in ambient air areregulated by law in the USA. A new, additional ambient air qualitystandard for PM-2.5 was introduced in the USA in 1997 as a result ofhealth studies that indicated a strong correlation between humanmortality and the concentration of fine particles below 2.5 μm.

Interest has now shifted towards nanoparticles generated by diesel andgasoline engines because they are understood to penetrate more deeplyinto human lungs than particulates of greater size and consequently theyare believed to be more harmful than larger particles, extrapolated fromthe findings of studies into particulates in the 2.5-10.0 μm range.

Size distributions of diesel particulates have a well-establishedbimodal character that corresponds to the particle nucleation andagglomeration mechanisms, with the corresponding particle types referredto as the nuclei mode and the accumulation mode respectively (see FIG.1). As can be seen from FIG. 1, in the nuclei mode, diesel PM iscomposed of numerous small particles holding very little mass. Nearlyall diesel particulates have sizes of significantly less than 1 μm, i.e.they comprise a mixture of fine, i.e. falling under the 1997 US law,ultrafine and nanoparticles.

Nuclei mode particles are believed to be composed mostly of volatilecondensates hydrocarbons, sulfuric acid, nitric acid etc) and containlittle solid material, such as ash and carbon. Accumulation modeparticles are understood to comprise solids (carbon, metallic ash etc.)intermixed with condensates and adsorbed material (heavy hydrocarbons,sulfur species, nitrogen oxide derivatives etc.). Coarse mode particlesare not believed to be generated in the diesel combustion process andmay be formed through mechanisms such as deposition and subsequentre-entrainment of particulate material from the walls of an enginecylinder, exhaust system, or the particulate sampling system. Therelationship between these modes is shown in FIG. 1.

The composition of nucleating particles may change with engine operatingconditions, environmental condition (particularly temperature andhumidity), dilution and sampling system conditions. Laboratory work andtheory have shown that most of the nuclei mode formation and growthoccur in the low dilution ratio range. In this range, gas to particleconversion of volatile particle precursors, like heavy hydrocarbons andsulfuric acid, leads to simultaneous nucleation and growth of the nucleimode and adsorption onto existing particles in the accumulation mode.Laboratory tests (see e.g. SAE 980525 and SAE 2001-01-0201) have shownthat nuclei mode formation increases strongly with decreasing airdilution temperature but there is conflicting evidence on whetherhumidity has an influence.

Generally, low temperature, low dilution ratios, high humidity and longresidence times favour nanoparticles formation and growth. Studies haveshown that nanoparticles consist mainly of volatile material like heavyhydrocarbons and sulfuric acid with evidence of solid fraction only atvery high loads.

Particulate collection of diesel particulates in a diesel particulatefilter is based on the principle of separating gas-borne particulatesfrom the gas phase using a porous barrier. Diesel filters can be definedas deep-bed filters and/or surface-type filters. In deep-bed filters,the mean pore size of filter media is bigger than the mean diameter ofcollected particles. The particles are deposited on the media through acombination of depth filtration mechanisms, including diffusionaldeposition (Brownian motion), inertial deposition (impaction) andflow-line interception (Brownian motion or inertia).

In surface-type filters, the pore diameter of the filter media is lessthan the diameter of the PM, so PM is separated by sieving. Separationis done by a build-up of collected diesel PM itself, which build-up iscommonly referred to as “filtration cake” and the process as “cakefiltration”.

It is understood that diesel particulate filters, such as ceramicwallflow monoliths, may work through a combination of depth and surfacefiltration: a filtration cake develops at higher soot loads when thedepth filtration capacity is saturated and a particulate layer startscovering the filtration surface. Depth filtration is characterized bysomewhat lower filtration efficiency and lower pressure drop than thecake filtration.

Selective catalytic reduction (SCR) of NO_(x) by nitrogenous compounds,such as ammonia or urea, was first developed for treating industrialstationary applications. SCR technology was first used in thermal powerplants in Japan in the late 1970s, and has seen widespread applicationin Europe since the mid-1980s. In the USA, SCR systems were introducedfor gas turbines in the 1990s and have been used more recently incoal-fired powerplants. In addition to coal-fired cogeneration plantsand gas turbines, SCR applications include plant and refinery heatersand boilers in the chemical processing industry, furnaces, coke ovens,municipal waste plants and incinerators. More recently, NO_(x) reductionsystems based on SCR technology are being developed for a number ofvehicular (mobile) applications in Europe, Japan, and the USA, e.g. fortreating diesel exhaust gas.

Several chemical reactions occur in an NH₃ SCR system, all of whichrepresent desirable reactions that reduce NO_(x) to nitrogen. Thedominant reaction is represented by reaction (1).4NO+4NH₃+O₂→4N₂+6H₂O  (1)

Competing, non-selective reactions with oxygen can produce secondaryemissions or may unproductively consume ammonia. One such non-selectivereaction is the complete oxidation of ammonia, shown in reaction (2).4NH₃+5O₂→4NO+6H₂O  (2)Also, side reactions may lead to undesirable products such as N₂O, asrepresented by reaction (3)4NH₃+5NO+3O₂→4N₂O+6H₂O  (3)

Various catalysts for promoting NH₃—SCR are known includingV₂O₅/WO₃/TiO₂ and transition metal/zeolites such as Fe/Beta (see U.S.Pat. No. 4,961,917) and transition metal/small pore zeolites (see WO2008/132452).

EP 1663458 discloses an SCR filter, wherein the filter is a wallflowmonolith and wherein an SCR catalyst composition permeates walls of thewallflow monolith. The specification discloses generally that the wallsof the wallflow filter can contain thereon or therein (i.e. not both)one or more catalytic materials. According to the disclosure,“permeate”, when used to describe the dispersion of a catalyst slurry onthe wallflow monolith substrate, means the catalyst composition isdispersed throughout the wall of the substrate.

WO 2008/136232 A1 discloses a honeycomb filter having a cell wallcomposed of a porous cell wall base material and, provided on its inflowside only or on its inflow and outflow sides, a surface layer andsatisfying the following requirements (1) to (5) is used as DPF: (1) thepeak pore diameter of the surface layer is identical with or smallerthan the average pore diameter of the cell wall base material, and theporosity of the surface layer is larger than that of the cell wall basematerial; (2) with respect to the surface layer, the peak pore diameteris from 0.3 to less than 20 μm, and the porosity is from 60 to less than95% (measured by mercury penetration method); (3) the thickness (L1) ofthe surface layer is from 0.5 to less than 30% of the thickness (L2) ofthe cell wall; (4) the mass of the surface layer per filtration area isfrom 0.01 to less than 6 mg/cm²; and (5) with respect to the cell wallbase material, the average pore diameter is from 10 to less than 60 11m, and the porosity is from 40 to less than 65%. See also SAE paper2009-01-0292.

NOx absorber catalysts (NACs) are known e.g. from U.S. Pat. No.5,473,887 and are designed to adsorb nitrogen oxides (NOx) from leanexhaust gas (lambda>1) and to desorb the NOx when the oxygenconcentration in the exhaust gas is decreased. Desorbed NOx may bereduced to N₂ with a suitable reductant, e.g. gasoline fuel, promoted bya catalyst component, such as rhodium, of the NAC itself or locateddownstream of the NAC. In practice, control of oxygen concentration canbe adjusted to a desired redox composition intermittently in response toa calculated remaining NOx adsorption capacity of the NAC, e.g. richerthan normal engine running operation (but still lean of stoichiometricor lambda=1 composition), stoichiometric or rich of stoichiometric(lambda<1). The oxygen concentration can be adjusted by a number ofmeans, e.g. throttling, injection of additional hydrocarbon fuel into anengine cylinder such as during the exhaust stroke or injectinghydrocarbon fuel directly into exhaust gas downstream of an enginemanifold.

A typical NAC formulation includes a catalytic oxidation component, suchas platinum, a significant quantity, i.e. substantially more than isrequired for use as a promoter such as a promoter in a TWC, of aNOx-storage component, such as barium, and a reduction catalyst, e.g.rhodium. One mechanism commonly given for NOx-storage from a leanexhaust gas for this formulation is:NO+½O₂→NO₂  (4); andBaO+NO₂+½O₂→Ba(NO₃)₂  (5),wherein in reaction (4), the nitric oxide reacts with oxygen on activeoxidation sites on the platinum to form NO₂. Reaction (5) involvesadsorption of the NO₂ by the storage material in the form of aninorganic nitrate.

At lower oxygen concentrations and/or at elevated temperatures, thenitrate species become thermodynamically unstable and decompose,producing NO or NO₂ according to reaction (6) below. In the presence ofa suitable reductant, these nitrogen oxides are subsequently reduced bycarbon monoxide, hydrogen and hydrocarbons to N2, which can take placeover the reduction catalyst (see reaction (5)).Ba(NO₃)₂→BaO+2NO+3/2O₂ or Ba(NO₃)₂→BaO+2NO₂+½O₂  (6); andNO+CO→½N₂+CO₂  (7);(Other reactions include Ba(NO₃)₂+8H₂→.BaO+2NH₃+5H₂O followed byNH₃+NO_(x)→N₂+yH₂O or 2NH₃+2O₂+CO→N₂+3H₂O+CO₂ etc.).

In the reactions of (4)-(7) above, the reactive barium species is givenas the oxide. However, it is understood that in the presence of air mostof the barium is in the form of the carbonate or possibly the hydroxide.The skilled person can adapt the above reaction schemes accordingly forspecies of barium other than the oxide and sequence of catalyticcoatings in the exhaust stream.

In Europe, since the year 2000 (Euro 3 emission standard) emissions aretested over the New European Driving Cycle (NEDC). This consists of fourrepeats of the previous ECE 15 driving cycle plus one Extra UrbanDriving Cycle (EUDC) with no 40 second warm-up period before beginningemission sampling. This modified cold start test is also referred to asthe “MVEG-B” drive cycle. All emissions are expressed in g/km.

The Euro 5/6 implementing legislation introduces a new PM mass emissionmeasurement method developed by the UN/ECE Particulate MeasurementProgramme (PMP) which adjusts the PM mass emission limits to account fordifferences in results using old and the new methods. The Euro 5/6legislation also introduces a particle number emission limit (PMPmethod), in addition to the mass-based limits.

Emission legislation in Europe from 1 Sep. 2014 (Euro 6) requirescontrol of the number of particles emitted from both diesel and gasolinepassenger cars. For diesel EU light duty vehicles the allowable limitsare: 500 mg/km carbon monoxide; 80 mg/km nitrogen oxides (NOx); 170mg/km total hydrocarbons+NOx; 4.5 g/km particulate matter (PM); andparticulate number standard of 6.0×10¹¹ per km. The presentspecification is based on the assumption that this number will beadopted in due course.

A difficulty in coating a filter with a catalyst composition is tobalance a desired catalytic activity, which generally increases withwashcoat loading, with the backpressure that is caused by the filter inuse (increased washcoat loading generally increases backpressure) andfiltration efficiency (backpressure can be reduced by adopting widermean pore size and higher porosity substrates at the expense offiltration efficiency).

SUMMARY OF THE INVENTION

We have now discovered, very surprisingly, that by coating a filtersubstrate monolith on a surface thereof with a washcoat, as opposed topermeating the filter walls with the washcoat as is disclosed in EP1663458 it is possible to achieve a beneficial balance of backpressure,filtration and catalytic activity. Moreover, we have found that byappropriate selection of molecular sieve size it is possible to tune thebackpressure of the filter at a similar catalytic activity, thusincreasing design options.

According to one aspect, the invention provides a filter for filteringparticulate matter (PM) from exhaust gas emitted from a compressionignition engine, which filter comprising a porous substrate having inletsurfaces and outlet surfaces, wherein the inlet surfaces are separatedfrom the outlet surfaces by a porous structure containing pores of afirst mean pore size, wherein the porous substrate is coated with awashcoat comprising a plurality of solid particles comprising amolecular sieve promoted with at least one transition metal wherein theporous structure of the washcoated porous substrate contains pores of asecond mean pore size, and wherein the second mean pore size is lessthan the first mean pore size.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully understood, reference ismade to the accompanying drawings wherein:

FIG. 1 is a graph showing the size distributions of PM in the exhaustgas of a diesel engine. For comparison, a gasoline size distribution isshown at FIG. 4 of SAE 1999-01-3530;

FIGS. 2A and 2B show schematic drawings of three embodiments ofwashcoated porous filter substrates according to the invention;

FIG. 3 is a schematic graph of mercury porosimetry relating the poresize distribution of a porous filter substrate, a porous washcoat layerand a porous filter substrate including a porous surface washcoat layer;

FIG. 4 is a graph showing the results of a Soot Loading Back Pressurestudy comparing backpressure against soot loading for 5.66 inch×6 inchSiC wallflow filters coated with two different oxidation catalystwashcoat loadings (g/in³) and a bare filter (all not according to theinvention) with a Fe/beta zeolite selective catalytic reduction (SCR)catalyst (according to the invention) at a comparable washcoat loading;

FIG. 5 is a graph comparing the backpressure in the same Soot LoadingBack Pressure test for a Cu/SSZ-13 zeolite (a small pore zeolite)catalyst and a Fe/Beta zeolite (a large pore zeolite) SCR catalyst; and

FIG. 6 is a bar chart comparing the particulate number emissions(particulate number per kilometer) from a 2.0 liter Euro 5 compliantlight duty diesel vehicle fitted with standard diesel oxidation catalystfollowed by a 3.0 liter SiC filter at 23 μm nominal mean pore sizecoated with a Fe/Beta zeolite SCR catalyst for meeting the Euro 5/6particle number emission limit of 6×10¹¹ km⁻¹ (UN/ECE ParticulateMeasurement Programme (PMP)) with the same system containing a barefilter.

DETAILED DESCRIPTION OF THE INVENTION

Mean pore size can be determined by mercury porosimetry.

It will be understood that the benefit of the invention is substantiallyindependent of the porosity of the substrate. Porosity is a measure ofthe percentage of void space in a porous substrate and is related tobackpressure in an exhaust system: generally, the lower the porosity,the higher the backpressure. However, the porosity of filters for use inthe present invention are typically >40% or >50% and porosities of45-75% such as 50-65% or 55-60% can be used with advantage. The meanpore size of the washcoated porous substrate is important forfiltration. So, it is possible to have a porous substrate of relativelyhigh porosity that is a poor filter because the mean pore size is alsorelatively high.

The porous substrate can be a metal, such as a sintered metal, or aceramic, e.g. silicon carbide, cordierite, aluminium nitride, siliconnitride, aluminium titanate, alumina, cordierite, mullite e.g., acicularmullite (see e.g. WO 01/16050), pollucite, a thermet such as Al₂O₃/Fe,Al₂O₃/N₁ or B₄C/Fe, or composites comprising segments of any two or morethereof. In a preferred embodiment, the filter is a wallflow filtercomprising a ceramic porous filter substrate having a plurality of inletchannels and a plurality of outlet channels, wherein each inlet channeland each outlet channel is defined in part by a ceramic wall of porousstructure, wherein each inlet channel is separated from an outletchannel by a ceramic wall of porous structure. This filter arrangementis also disclosed in SAE 810114, and reference can be made to thisdocument for further details. Alternatively, the filter can be a foam,or a so-called partial filter, such as those disclosed in EP 1057519 orWO 01/080978.

In one embodiment, the first mean pore size e.g. of surface pores of theporous structure of the porous filter substrate is from 8 to 45 μm, forexample 8 to 25 μm, 10 to 20 μm or 10 to 15 μm. In particularembodiments, the first mean pore size is >18 μm such as from 15 to 45μm, 20 to 45 μm e.g. 20 to 30 μm, or 25 to 45 μm.

In embodiments, the filter has a washcoat loading of >0.25 g in⁻³, suchas >0.50 g in⁻³ or ≧0.80 g in⁻³, e.g. 0.80 to 3.00 g in⁻³. In preferredembodiments, the washcoat loading is >1.00 g in⁻³ such as ≧1.2 gin⁻³, >1.5 g in⁻³, >1.6 g in⁻³ or >2.00 g in⁻³ or for example 1.6 to 2.4g in⁻³. In particular combinations of filter mean pore size and washcoatloading the filter combines a desirable level of particulate filtrationand catalytic activity at acceptable backpressure.

In a first, preferred embodiment, the filter comprises a surfacewashcoat, wherein a washcoat layer substantially covers surface pores ofthe porous structure and the pores of the washcoated porous substrateare defined in part by spaces between the particles (interparticlepores) in the washcoat. That is, substantially no washcoat enters theporous structure of the porous substrate. Methods of making surfacecoated porous filter substrates include introducing a polymer, e.g. polyvinyl alcohol (PVA), into the porous structure, applying a washcoat tothe porous filter substrate including the polymer and drying, thencalcining the coated substrate to burn out the polymer. A schematicrepresentation of the first embodiment is shown in FIG. 2A.

Methods of coating porous filter substrates are known to the skilledperson and include, without limitation, the method disclosed in WO99/47260, i.e. a method of coating a monolithic support, comprising thesteps of (a) locating a containment means on top of a support, (b)dosing a pre-determined quantity of a liquid component into saidcontainment means, either in the order (a) then (b) or (b) then (a), and(c) by applying pressure or vacuum, drawing said liquid component intoat least a portion of the support, and retaining substantially all ofsaid quantity within the support. Such process steps can be repeatedfrom another end of the monolithic support following drying of the firstcoating with optional firing/calcination.

In this first embodiment, an average interparticle pore size of theporous washcoat is 5.0 nm to 5.0 μm, such as 0.1-1.0 μm.

A D90 of solid washcoat particles in this first surface coatingembodiments can be greater than the mean pore size of the porous filtersubstrate and can be in the range 10 to 40 μm, such as 15 to 30 μm or 12to 25 μm. “D90” as used herein defines the particle size distribution ina washcoat wherein 90% of the particles present have a diameter withinthe range specified. Alternatively, in embodiments, the mean size of thesolid washcoat particles is in the range 1 to 20 μm. It will beunderstood that the broader the range of particle sizes in the washcoat,the more likely that washcoat may enter the porous structure of theporous substrate. The term “substantially no washcoat enters the porousstructure of the substrate” should therefore be interpreted accordingly.

According to a second embodiment, the washcoat can be coated on inletand/or outlet surfaces and also within the porous structure of theporous substrate. We believe that a surface coating around a poreopening at the inlet and/or outlet surfaces, thereby narrowing the e.g.surface pore size of a bare filter substrate, promotes interaction ofthe gas phase including PM without substantially restricting the porevolume, so not giving rise to significant increases in back pressure.That is, the pores at a surface of the porous structure comprise a poreopening and the washcoat causes a narrowing of substantially all thepore openings. A schematic representation of the second embodiment isshown in FIG. 2B.

Methods of making a filter according to the second embodiment caninvolve appropriate formulation of the washcoat known to the personskilled in the art including adjusting viscosity and surface wettingcharacteristics and application of an appropriate vacuum followingcoating of the porous substrate (see also WO 99/47260).

In our research and development work we have found that coated filtersaccording to the first or second embodiments can be obtained by dipcoating in a washcoat composition followed by draining the coated part,then application of a low vacuum to remove excess washcoat before dryingand calcining. This method produces a surface coating (as determined byscanning electron microscope (SEM)) and in this respect distinguishesthe coated filter wherein the SCR catalyst “permeates” the filter walls,as disclosed in EP 1663458.

In the first and second embodiments, wherein at least part of thewashcoat is coated on inlet and/or outlet surfaces of the poroussubstrate, the washcoat can be coated on the inlet surfaces, the outletsurfaces or on both the inlet and the outlet surfaces. Additionallyeither one or both of the inlet and outlet surfaces can include aplurality of washcoat layers, wherein each washcoat layer within theplurality of layers can be the same or different, e.g. the mean poresize in a first layer can be different from that of a second layer. Inembodiments, washcoat intended for coating on outlet surfaces is notnecessarily the same as for inlet surfaces.

Where both inlet and outlet surfaces are coated, the washcoatformulations can be the same or different. Where both the inlet and theoutlet surfaces are washcoated, the mean pore size of washcoat on theinlet surfaces can be different from the mean pore size of washcoat onthe outlet surfaces. For example, the mean pore size of washcoat on theinlet surfaces can be less than the mean pore size of washcoat on theoutlet surfaces. In the latter case, a mean pore size of washcoat on theoutlet surfaces can be greater than a mean pore size of the poroussubstrate.

Whilst it is possible for the mean pore size of a washcoat applied toinlet surfaces to be greater than the mean pore size of the poroussubstrate, it is advantageous to have washcoat having smaller pores thanthe porous substrate in washcoat on inlet surfaces to prevent or reduceany combustion ash or debris entering the porous structure.

In the second embodiment, wherein at least part of the washcoat is inthe porous structure, a size, e.g. a mean size, of the solid washcoatparticles can be less than the mean pore size of the porous filtersubstrate for example in the range 0.1 to 20 μm, such as 1 to 18 μm, 1to 16 μm, 2 to 15 μm or 3 to 12 μm. In particular embodiments, theabovementioned size of the solid washcoat particles is a D90 instead ofa mean size.

In further particular embodiments, the surface porosity of the washcoatis increased by including voids therein. Exhaust gas catalysts havingsuch features are disclosed, e.g. in our WO 2006/040842 and WO2007/116881.

By “voids” in the washcoat layer herein, we mean that a space exists inthe layer defined by solid washcoat material. Voids can include anyvacancy, fine pore, tunnel-state (cylinder, prismatic column), slitetc., and can be introduced by including in a washcoat composition forcoating on the filter substrate a material that is combusted duringcalcination of a coated filter substrate, e.g. chopped cotton ormaterials to give rise to pores made by formation of gas ondecomposition or combustion.

The average void ratio of the washcoat can be from 5-80%, whereas theaverage diameter of the voids can be from 0.2 to 500 μm, such as 10 to250 μm.

Promoter metals can be selected from the group consisting of at leastone of Cu, Hf, La, Au, In, V, lanthanides and Group VIII transitionmetals, such as Fe. The molecular sieve for use in the present inventioncan be an aluminosilicate zeolite, a metal-substituted aluminosilicatezeolite or a non-zeolitic molecular sieve. Metal substituted molecularsieves with application in the present invention include those havingone or more metals incorporated into a framework of the molecular sievee.g. Fe in-framework Beta and Cu in-framework CHA.

Where the molecular sieve is non-zeolitic molecular sieve, it can be analuminophosphate molecular sieve selected from the group consisting ofaluminophosphate (AlPO) molecular sieves, metal substitutedaluminophosphate molecular sieves (MeAlPO) zeolites,silico-aluminophosphate (SAPO) molecular sieves and metal substitutedsilico-aluminophosphate (MeAPSO) molecular sieves.

In particular, the molecular sieve can be a small, medium or large poremolecular sieve. By “small pore molecular sieve” herein we mean amolecular sieve containing a maximum ring size of 8, such as CHA; by“medium pore molecular sieve” herein we mean a molecular sievecontaining a maximum ring size of 10, such as ZSM-5; and by “large poremolecular sieve” herein we mean a molecular sieve having a maximum ringsize of 12, such as beta. Small pore molecular sieves with particularapplication in the present invention are any of those listed in Table 1of WO2008/132452.

Specific examples of useful molecular sieves are selected from the groupconsisting of AEI, ZSM-5, ZSM-20, ERI, LEV, mordenite, BEA, Y, CHA,MCM-22 and EU-1.

The metal substitutent and/or the transition metal promoter can beselected from the group consisting of groups IB, IIB, IIIA, IIIB, VB,VIB, VIB and VIII of the periodic table.

In embodiments, the metal can be selected from the group consisting ofCr, Co, Cu, Fe, Hf, La, Ce, In, V, Mn, Ni, Zn, Ga and the preciousmetals Ag, Au, Pt, Pd and Rh.

Metals of particular interest for use as transition metal promoters inso-called NH₃—SCR are selected from the group consisting of Ce, Fe andCu. Suitable nitrogenous reductants include ammonia. Ammonia can begenerated in situ e.g. during rich regeneration of a NAC disposedupstream of the filter (see the alternatives to reactions (6) and (7)hereinabove). Alternatively, the nitrogenous reductant or a precursorthereof can be injected directly into the exhaust gas. Suitableprecursors include amrnomum formate, urea and amrnomum carbamate.Decomposition of the precursor to ammonia and other by-products can beby hydrothermal or catalytic hydrolysis.

According to a further aspect, the invention provides an exhaust systemfor a compression ignition engine, which system comprising a filteraccording to the invention. Compression ignition engines for use in thisaspect of the invention can be fuelled by diesel fuel, biodiesel fuel,blends of diesel and biodiesel fuels and compressed natural gas.

In one embodiment, the exhaust system comprises means for injecting anitrogenous reductant or a precursor thereof, into exhaust gas upstreamof the filter. In a particular embodiment, the nitrogenous reductant isa fluid.

In another aspect, the invention provides a compression ignition enginecomprising an exhaust system according to the invention.

In a further aspect, the invention provides a method of trappingparticulate matter (PM) from exhaust gas emitted from a compressionignition engine by depth filtration, which method comprising contactingexhaust gas containing the PM with a filter comprising a poroussubstrate having inlet and outlet surfaces, wherein the inlet surfacesare separated from the outlet surfaces by a porous structure containingpores of a first mean pore size, wherein the porous substrate is coatedwith a washcoat comprising a plurality of solid particles comprising amolecular sieve promoted with at least one metal wherein the porousstructure of the washcoated porous substrate contains pores of a secondmean pore size, and wherein the second mean pore size is less than thefirst mean pore size.

In a further aspect, the invention provides a method of adjusting filterbackpressure in an exhaust system of a compression ignition engine bycoating the filter with a first transition metal promoted molecularsieve SCR catalyst, testing the filter backpressure to determine whetherit meets a pre-determined backpressure requirement and selecting asecond transition metal promoted molecular sieve SCR catalyst in orderto reduce the backpressure in the system containing the filter coatedwith the first transition metal promoted molecular sieve SCR catalyst,wherein the pore size of the second molecular sieve is > the firstmolecular sieve.

FIGS. 2A and 2B show a cross-section through a porous filter substrate10 comprising a surface pore 12. FIG. 2A shows a first embodiment,featuring a porous surface washcoat layer 14 comprised of solid washcoatparticles, the spaces between which particles define pores(interparticle pores). It can be seen that the washcoat layer 14substantially covers the pore 12 of the porous structure and that a meanpore size of the interparticle pores 16 is less than the mean pore size12 of the porous filter substrate 10.

FIG. 2B shows a second embodiment comprising a washcoat that is coatedon an inlet surface 16 and additionally within a porous structure 12 ofthe porous substrate 10. It can be seen that the washcoat layer 14causes a narrowing of a pore openings of surface pore 12, such that amean pore size 18 of the coated porous substrate is less than the meanpore size 12 of the porous filter substrate 10.

FIG. 3 shows an illustration of a graph relating pore size to porenumber for a porous filter substrate 20, a porous washcoat layer 22 anda porous diesel filter substrate including a surface washcoat layer 24.It can be seen that the filter substrate has a mean pore size of theorder of about 15 μm. The washcoat layer has a bimodal distributioncomprised of intraparticle pores 22A (at the nanometer end of the range)and interparticle pores 22B towards the micrometer end of the scale. Itcan also be seen that by coating the porous filter substrate with awashcoat according to the invention that the pore distribution of thebare filter substrate is shifted in the direction of the interparticlewashcoat pore size (see arrow).

EXAMPLES

The following Examples are provided by way of illustration only. In theExamples, the Soot Loading Back Pressure (“SLBP”) test uses theapparatus and method described in EP 1850068, i.e.:

-   -   (i) an apparatus for generating and collecting particulate        matter derived from combusting a liquid carbon-containing fuel,        which apparatus comprising a fuel burner comprising a nozzle,        which nozzle is housed in a container, which container        comprising a gas inlet and a gas outlet, said gas outlet        connecting with a conduit for transporting gas from the gas        outlet to atmosphere, means for detecting a rate of gas flowing        through the gas inlet and means for forcing an oxidising gas to        flow from the gas inlet via the container, the gas outlet and        the conduit to atmosphere, a station for collecting particulate        matter from gas flowing through the conduit and means for        controlling the gas flow-forcing means in response to a detected        gas flow rate at the gas inlet, whereby the rate of gas flow at        the gas inlet is maintained at a desired rate to provide        substoichiometric fuel combustion within the container, thereby        to promote particulate matter formation; and    -   (ii) a method of generating and collecting particulate matter        derived from combusting liquid carbon-containing fuel in an        oxidising gas, which method comprising burning the fuel in a        substoichiometric quantity of oxidising gas in a fuel burner,        said fuel burner comprising a nozzle, which nozzle being housed        in a container, forcing an oxidising gas to flow from a gas        inlet to the container to atmosphere via a gas outlet to the        container and a conduit connected to the gas outlet, collecting        particulate matter at a station located within the conduit,        detecting a rate of oxidising gas flow at the gas inlet and        controlling the rate of oxidising gas flow so that a desired        rate of oxidising gas flow is maintained at the gas inlet.

The filter is inserted in the station for collecting particulate matterfrom gas flowing through the conduit. The fresh filter is firstpre-conditioned at an air flow rate 80 kg/hr in a lean burn combustionstream using low sulphur diesel fuel (10 ppm S) to raise the filterinlet temperature to 650° C., a temperature that is typically used on avehicle to regenerate a soot-loaded filter. This pre-conditioning steptemperature is well above the soot combustion temperature and is toensure that the filter on test is clean at the outset. Pressure sensorsdisposed upstream and downstream of the station monitor the backpressureacross the filter.

The backpressure against time is plotted in the accompanying FIGS. 4-6.The SLBP test is carried out at a filter inlet temperature of 250° C. atair flow rate of 180 kg/hour combusting low sulphur diesel fuel (10 ppmS).

Example 1 CSF and SCR Catalyst Coated Filter Backpressure Comparison

Three commercially available uncoated 5.66 inch×6 inch SiC wallflowfilters having 60% porosity and a mean pore size of 20-25 μm were eachcoated, separately, with a catalyst washcoat for a catalysed soot filter(CSF) comprising precious metal supported on an alumina-based metaloxide and an Cu/Beta zeolite selective catalytic reduction (SCR)catalyst coating. The CSF coating was obtained according to the methoddisclosed in WO 99/47260, i.e. a method of coating a monolithic support,comprising the steps of (a) locating a containment means on top of asupport, (b) dosing a pre-determined quantity of a liquid component intosaid containment means, either in the order (a) then (b) or (b) then(a), and (c) by applying pressure or vacuum, drawing said liquidcomponent into at least a portion of the support, and retainingsubstantially all of said quantity within the support. The coatedproduct was dried and calcined and then the process steps were repeatedfrom another end of the wallflow filter. The SCR coated filter wasobtained by dip coating followed by draining, the application of a lowvacuum to remove excess washcoat before drying and calcining. Thismethod produces a surface coating (as determined by scanning electronmicroscope (SEM)) and in this respect distinguishes the coated filterwherein the SCR catalyst “permeates” the filter walls, as disclosed inEP 1663458. Two different CSF washcoat loadings were obtained, at 0.6g/in³ and 1.2 g/in³. The SCR coated filter was washcoated at a loadingof at 1.1 g/in³.

The three coated filters were tested using the SLBP test, a fourth,uncoated filter was used as a control. The results are shown in FIG. 4,from which it can be seen that the CSF coating at approximately the samewashcoat loading has considerably higher backpressure compared to theSCR coated filter. We conclude, therefore, that there is an inherentcoating porosity difference between CSF and SCR coated filter.

Example 2 SCR Catalyst Coated Filter Backpressure Comparison

Identical commercially available 5.66 inch×7.5 inch SiC wallflow filtershaving 60% porosity and a mean pore size of 20-25 μm were washcoated toa loading of 1.1 g/in³ with Cu/SSZ-13 zeolite and Cu/Beta zeolite SCRcatalysts, each catalyst having the same particle size D90 (90% ofparticles in washcoat having a particle size) at between 4.8-5 μm butapart from the transition metal/zeolite were in all other respects weresubstantially identical. The method of manufacture was to dip coat thepart followed by draining, the application of a low vacuum to removeexcess washcoat and then drying and calcining. A SLBP test was done tocompare the finished parts.

The results are presented in FIG. 5, from which it can be seen that thefilter coated with the Cu/Beta zeolite catalyst has a lower rate ofbackpressure increase than the filter coated with the Cu/SSZ-13 zeolitecatalyst. Since the fundamental difference between the two SCR catalystsis that the pore size of the SSZ-13 zeolite is 3.8×3.8 Angstroms and5.6-7.7 Angstroms for the Beta zeolite (source: Structure Commission ofthe International Zeolite Association), we conclude that it is possibleto adjust backpressure in the exhaust system, thereby increasing designoptions, by selecting a molecular sieve-based SCR catalyst having anappropriate pore size to achieve the desired backpressure objective andat the same time meeting emission standards for NOx.

Example 3 Vehicle Testing

A 3.0 liter capacity SiC filter at 58% porosity and 23 μm nominal meanpore size Cu/Beta zeolite SCR catalyst coated filter manufactured by thedip coating method described in Example 1 was inserted into an exhaustsystem of a 2.0 liter Euro 5 compliant light duty diesel vehicle behinda standard diesel oxidation catalyst. The vehicle containing the fresh(i.e. unaged) catalysed filter was then driven over the MVEG-B drivecycle, then the EUDC part of the MVEG-B cycle three times consecutivelyto pre-condition the filter.

In Europe, since the year 2000 (Euro 3 emission standard) emissions aretested over the New European Driving Cycle (NEDC). This consists of fourrepeats of the previous ECE 15 driving cycle plus one Extra UrbanDriving Cycle (EUDC) with no 40 second warm-up period before beginningemission sampling. This modified cold start test is also referred to asthe “MVEG-B” drive cycle. All emissions are expressed in g/km.

The Euro 5/6 implementing legislation introduces a new PM mass emissionmeasurement method developed by the UN/ECE Particulate MeasurementProgramme (PMP) which adjusts the PM mass emission limits to account fordifferences in results using old and the new methods. The Euro 5/6legislation also introduces a particle number emission limit (PMPmethod), in addition to the mass-based limits. The new Euro 5/6 particlenumber emission limit of 6×10¹¹ km⁻¹ using the PMP protocol allows forpre-conditioning of the system prior testing the system to determinewhether it meets the emission standard over the MVEG-B drive cycle.

Repeated cold MVEG-B cycles were then run using the pre-conditionedsystem. The coated filter was exchanged in the system for an uncoatedfilter as a control. The results are shown as a bar chart in FIG. 6comparing the particulate number emissions (particulate number perkilometer) from which it can be seen that despite pre-conditioning,which would be expected to develop a soot cake providing improvedfiltration, the uncoated filter initially failed the particle numberemission limit of 6×10⁻¹¹ km⁻¹, but with repeated drive cycles theparticle number came down consistently to within the emission standard.By contrast it can be seen that the coated filter is well within theemission standard from the first drive cycle following pre-conditioning.We interpret these data to mean that the coated filter promotes sootcaking that improves diesel particulate filtration and therefore a moreimmediate reduction in particle number, yet—as is seen in Example 2—theCu/Beta zeolite coated filter provides a lower backpressure comparedwith the Cu/SSZ-13 zeolite SCR catalyst or a CSF coating at a similarwashcoat loading (see Example 1). Accordingly, the surface Cu/Beta SCRcatalyst coating takes away the requirement to have a soot layer on ahigher porosity/mean pore size filter before filtration occurs.Accordingly, the invention provides benefits for particle numberreduction in “real world” driving conditions, as opposed to theidealised drive cycle conditions set for meeting emission standards.

For the avoidance of any doubt, the entire contents of all prior artdocuments cited herein is incorporated herein by reference.

The invention claimed is:
 1. A filter for removing particulate matterfrom exhaust gas comprising: a. a filter comprising a porous substratehaving inlet surfaces and outlet surfaces, wherein the inlet surfacesare separated from the outlet surfaces by the porous substrate; b. afirst catalyst washcoat on the inlet surfaces, wherein the firstcatalyst washcoat has a mean particle size greater than the mean poresize of the filter; c. a second catalyst washcoat within the poroussubstrate, wherein the second catalyst washcoat has a mean particle sizeless than the mean pore size of the porous filter.
 2. The filter ofclaim 1, wherein the first catalyst washcoat has a mean particle size of10 to 40 μm.
 3. The filter of claim 1, wherein the second catalystwashcoat has a mean particle size of 0.1 to 20 μm.
 4. The filter ofclaim 1, wherein the first catalyst washcoat has a D₉₀ particle size of10 to 40 μm second catalyst washcoat has a D₉₀ particle size of 3 to 12μm.
 5. The filter of claim 1, wherein a least one of the first catalystwashcoat and the second catalyst washcoat comprise platinum and/orpalladium on a high surface area support.
 6. The filter of claim 1,wherein filter further comprises a third catalyst washcoat on the outletsurface.